In case you hadn’t already marked your calendar, Thursday, May 25, is National Wine Day. Before you get too carried away celebrating, there are a few facts you should know about how alcohol consumption (wine or anything else) really affects your health. The latest evidence suggests it may help your heart, but it might also raise your risk of cancer slightly.

What is clear is that drinking in moderation is absolutely key for wine to be at all healthful.

The Benefits

No matter which studies you look at, any purported benefits associated with drinking are related specifically to “moderate” consumption: one drink per day for women and up to two for men. (Men are allowed more to account for their generally larger size and differences in the way they metabolize, or break down, alcohol.)

For wine, one drink is 5 ounces. And because wine glasses come in so many different shapes and sizes, it can be tough to judge just when to stop when you’re pouring wine. The image below shows you just what 5 ounces of wine looks like in a variety of glass sizes.

If you stick to those amounts, the evidence is pretty clear that alcohol can boost your heart health. “The association between moderate alcohol intake and lower risk of myocardial infarction [heart attack] has been studied in well-designed observational studies for nearly 50 years,” says Kenneth Mukamal, M.D., M.P.H., a professor of medicine at Harvard Medical School who has researched this topic extensively.

And the studies have been pretty consistently positive over the decades. One from 2003 found that men who were moderate drinkers were 30 to 35 percent less likely to have a heart attack than those who didn’t drink at all. Data released last year from the long-running Nurses' Health Study showed that moderate drinking was associated with a 23 percent lower risk of heart disease in women.

“Alcohol affects platelets, acting like a mild blood thinner,” Mukamal explains. Moderate alcohol consumption also raises HDL cholesterol (the “good” kind) and lowers levels of the blood component fibrinogen, which may also help keep blood thinner.

The Risks

Alcohol has been linked to a small increased risk of cancer in general. A 2015 study published in the British Medical Journal (BMJ) found that compared with nondrinkers, moderate drinkers had a 2 to 6 percent higher risk.

But the association between moderate alcohol intake and the risk of breast cancer was stronger. Women who drank the amount of alcohol in one-third to one glass of wine per day had a 13 percent increased risk of cancer, mostly driven by breast cancer.

The body metabolizes alcohol into various substances. "Acetaldehyde, the first and most toxic alcohol metabolite, is considered a cancer-causing agent,” says Yin Cao, Sc.D., an instructor of medicine at Massachusetts General Hospital and Harvard Medical School, and lead author of the BMJ study. “And breast tissue may be more susceptible to alcohol than other organs.” Alcohol might damage the DNA in breast cells. It also increases the amount of estrogen circulating in the body, and higher levels of that hormone are associated with some types of breast cancer.

Once drinking goes beyond the small amounts defined as moderate, the risks quickly start to outweigh any potential cardiovascular benefits. Higher levels of alcohol intake are linked to increases in heart disease, high blood pressure, heart attack, and stroke, as well as the development of various types of cancer.

The Unknowns

The data on alcohol is very consistent, but most of the studies don't prove cause and effect. Many are observational—that is, they look at what people do in their real lives rather than randomly assign people to drink or abstain from alcohol, then follow them to see the effects on their health. The latter type of study would be ideal, but creating a placebo for alcohol to give to a control group is tricky. And attempting to separate large groups of subjects and randomly instruct them to drink or not drink for several years has proved to be almost impossible.

Without that clinical evidence, some experts are reluctant to recommend moderate drinking as a health tactic. “The observational data is good, but what limited experimental evidence we have shows no benefit to moderate drinking,” says Michael Criqui, M.D., M.P.H., a professor in the division of preventive medicine at the University of California at San Diego. Therefore, the American Heart Association and other health organizations advise that if you do drink alcohol, do so in moderation. And if you don’t, you shouldn’t start.  

Is There Something Special About Red Wine?

The short answer is, we don’t know for sure. “In all of the population studies on the effects of moderate drinking, we’ve seen no clear difference between types of alcohol,” Mukamal says. But wine drinking is linked to other healthy behavior—such as maintaining a better diet and getting regular exercise. Wine is also typically consumed in a healthier manner—with a meal—unlike beer or liquor.

As for the much-touted resveratrol (an antioxidant) in red wine, the evidence is mixed at best. Most of the studies have been done in animals, and the ones that show any benefit usually involve extremely high doses of resveratrol. “You would need to drink liters of wine a day to get those levels,” Mukamal says.

Bottom line: As long as you raise just one glass of wine (or two if you’re a guy), the health-related risks are pretty small. But rather than kid yourself you’re doing something super healthy, just sip and enjoy—in moderation.